Lead-on-chip (LOC) semiconductor devices are gaining widespread acceptance in the semiconductor industry, particularly in the area of semiconductor memories. LOC devices are resin encapsulated devices having leads attached directly above an active surface of a semiconductor die. Thus, a lead frame for LOC devices does not include a flag or die mounting paddle. Instead, the die is adhesively bonded directly to the leads. Once attached to the die, the leads are wire bonded to corresponding bonding pads on the active surface of the die. In some devices, bonding pads are located along a centerline of the die whereas in other devices peripheral bonding pads are used.
An on-going problem with many semiconductor devices, including some LOC devices, is that of simultaneous switching noise. Frequent current changes from switching circuits in a device results in fluctuations or transients in the device's power distribution system. As the switching speeds increase and as the number of circuits increases, the problem of noise is worsened. Since semiconductor manufacturers are continually striving for denser and faster circuits, switching noise is a serious and continuing problem.
The most accepted way to suppress the effects of simultaneous switching noise is to use a decoupling capacitor coupled to ground and power distributions of a device. The decoupling capacitor stores charge which would otherwise distort power distribution. While decoupling capacitors are more or less the accepted way to reduce switching noise, the manner in which a decoupling capacitor is implemented is quite varied. One method of using a decoupling capacitor is to use a capacitor which is external to the device itself, or in other words one which is located outside of a package body. A problem with external decoupling capacitors is that the distance between the capacitor and the die limits the reduction in inductance, thereby limiting the reduction of switching noise. Ideally, a decoupling capacitor is located as close to a die as possible. Another disadvantage with external decoupling capacitors is that external devices occupy board space which could otherwise be used for active components. Device density on a user board is of critical importance in achieving a compact product, so many users are reluctant to employ external decoupling capacitors.
Decoupling capacitors other than external capacitors are also known in the semiconductor industry. For instance, "close-attach" decoupling capacitors are discrete ceramic decoupling capacitors mounted directly to a semiconductor die and electrically coupled to ground and power bonding pads of the die. The die and decoupling capacitor are encased in a cavity type package. Another type of decoupling capacitor used in the industry is a decoupling capacitor attached below a flag or die paddle of a conventional metal lead frame. These decoupling capacitors typically include electrodes comprised of the lead frame material (e.g. copper or iron-nickel alloys) and a ceramic dielectric layer between the electrodes. The electrodes include tabs which are bonded to power and ground leads of the lead frame.
Each of the known decoupling capacitors described above are not particularly suitable for use in LOC devices. Since there is no flag or die paddle in an LOC device, a decoupling capacitor cannot be attached thereto. While it is possible to incorporate a decoupling capacitor beneath a die in an LOC device in a manner similar to attaching a decoupling capacitor beneath a flag, this would undesirably increase the profile or thickness of the device. Also, a close-attach capacitor is not suitable because there is no room on a die's active surface in an LOC device because the leads extend across the active surface. An external capacitor can be used in conjunction with an LOC device, but as circuit density and switching speeds increase, the effectiveness of an external decoupling capacitor is diminished.